Physical, Rheological, and Morphological Properties of Asphalt Reinforced by Basalt Fiber and Lignin Fiber

materials

Article Physical, Rheological, and Morphological Properties of Asphalt Reinforced by Basalt Fiber and Lignin Fiber

Changjiang Kou 1,2,* , Xing Wu 1, Peng Xiao 1, Yang Liu 2 and Zhengguang Wu 1

1 College of Civil Science and Engineering, Yangzhou University, Yangzhou 225127, China; [email protected] (X.W.); [email protected] (P.X.); [email protected] (Z.W.) 2 Centre for Pavement and Transportation Technology, University of Waterloo, Waterloo, ON N2L 3G1, Canada; [email protected] * Correspondence: [email protected]; Tel.: +86-1586-132-0609

Received: 16 May 2020; Accepted: 30 May 2020; Published: 1 June 2020

Abstract: Studies show that each kind of fiber has its own advantages in improving the properties of asphalt binders. However, there are very limited research studies about mixed fiber-reinforced asphalt (MFRA). In this study, two kinds of fibers, basalt fiber (BF) and lignin fiber (LF), were selected to reinforce SBS (styrene–butadiene–styrene triblock copolymer)-modified asphalt, which is now widely used in pavement engineering. MFRA samples with different fiber mix ratios (FMRs) were prepared for the tests of softening point, ductility, and rheological properties, the micromorphology of which was studied by using scanning electron microscope (SEM). The oil (asphalt) absorption rates of mixed fibers with different FMRs were also tested. The results show that the properties of MFRA were affected by the physical and chemical properties of fibers. Basalt fiber can better strengthen the physical properties of MFRA, while lignin fiber is good for improving the rheological properties, and the oil absorption rate of lignin fiber is higher than that of basalt fiber. Furthermore, the best FMR calculated by the efficacy coefficient method (ECM) was recommended as 1:2 (BF:LF). An interface layer between the fiber and asphalt was observed from the micro images, proving that the fibers bond well with the asphalt. Generally, mixing BF and LF together into SBS-modified asphalt could make full use of the advantages of different fibers and reinforce the comprehensive performance of MFRA better.

Keywords: mixed fiber-reinforced asphalt; fiber mix ratio; basalt fiber; lignin fiber; comprehensive performance

1. Introduction With the rapid development of the modern economy, more and more fibers are being used in scientific research and engineering practice. Using fibers as reinforcements can often have better economic and performance advantages in extending the service life of matrix materials [1–3]. In the field of road engineering, many scholars have done a lot of research on the asphalt mixtures reinforced by different fibers, and found that basalt fiber (BF) and lignin fiber (LF) can significantly enhance the performance of asphalt and asphalt mixtures [4–6]. As a kind of composite material, the performance of asphalt mixture depends largely on its composition [7]. In asphalt mixtures, fibers are mainly combined with asphalt, and they together play a very important role in the asphalt mixtures. There are some evidences showing that the impact of fiber on asphalt has a good correlation with the impact on asphalt mixture [8–10]. Scholars around the world have begun to pay attention to the theoretical study of the performance of fiber-reinforced asphalt mortar [11–13].

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Here are some details of several studies on the fiber-reinforced asphalt and asphalt mixtures reinforced by BF or LF. Cheng et al. [14] studied the effect of basalt fiber and diatomite on the performance of asphalt mixtures and found that the addition of basalt fiber is the main reason for improving the performance of asphalt mixture. Punya et al. [15] found that the addition of basalt fiber can reduce the penetration value of asphalt and increase the softening value. Wei et al. [16] studied the modification effect of basalt fiber on asphalt, and it was found that the rutting resistance property of SBS-modified asphalt increased significantly after basalt fiber was added. Wang et al. [17] found that basalt fiber could significantly reduce the damage degree of asphalt mixture by about 25% through the freeze–thaw cycle test. Fu et al. [18] used anti-rutting agent and lignin fiber to improve the performances of asphalt mixtures. After adopting these two additives, the high and low temperature and water stabilities were improved obviously. Xu et al. [3] investigated the reinforcing effects of lignin fiber and other three kinds of fibers on the asphalt mixtures and found that fibers can significantly improve the rutting resistance, fatigue life, and stiffness of the asphalt mixtures. Many studies have focused on the influence of single fiber on the performance of asphalt mortar or mixture or the comparison of the influences of multiple fibers on the performance of asphalt mortar or mixture. In fact, adding mixed fibers into the asphalt could make full use of the advantages of different fibers and get a better reinforcement effect on the asphalt. The addition of different fibers will have a synergic effect on the asphalt, making the mixed fiber-reinforced asphalt (MFRA) have all the advantages of the corresponding single-fiber reinforced asphalt binders. In the meantime, different fibers will form a more complex system in the MFRA, which will make it more stable. However, the study about the performance or the strengthening mechanism of mixed fiber-reinforced asphalt (MFRA) was ignored. Therefore, this paper considers adding BF and LF together into SBS (styrene–butadiene–styrene triblock copolymer) modified asphalt to study the performance and strengthening mechanism of mixed fiber-reinforced asphalt (MFRA). The MFRA samples were designed using several different fiber mix ratios (FMRs). Tests of softening point and ductility were chosen to study the high and low temperature physical properties of MFRA; a dynamic rheology test was chosen to study the rheological properties of MFRA; and the Scanning Electron Microscope (SEM) test was chosen to observe the micrograph of MFRA to explain the strengthening mechanism of MFRA. The conclusions of this study have certain meanings for the design of fiber-reinforced asphalt. As the influence of fiber on asphalt has a good correlation with the influence on asphalt mixture [8,9], this study is not only of great significance to guide the design of fiber-reinforced asphalt but also of great importance on the design of fiber-reinforced asphalt mixture.

2. Materials and Methods

2.1. Materials

2.1.1. Asphalt In this paper, the SBS-modiﬁed asphalt was provided by Jiangsu Tiannuo Road Materials Technology Co., Ltd., Zhenjiang, China. The properties of it are listed in Table1.

Table 1. Properties of styrene–butadiene–styrene (SBS) modiﬁed asphalt.

Index Value

Penetration (25 ◦C, 100 g, 5 s), 0.1 mm 59.5 Softening point (TR&B), ◦C 80.0 Ductility (5 ◦C, 5 cm/min), cm 37.9 Solubility, % 99.9 Elastic recovery (25 ◦C), % 98.7 Rotational viscosity (135 C), Pa s 2.302 ◦ · Relative density (25 ◦C) 1.031 Materials 2020, 13, 2520 3 of 17

2.1.2.Materials Basalt 2020,, Fiber 13,, xx FORFOR and PEERPEER Lignin REVIEWREVIEW Fiber 3 of 17 Basalt fiber (BF) is produced from natural basalt at high temperature [19], and the manufacturing Basalt fiber (BF) is produced from natural basalt at high temperature [19], and the manufacturing process is very environmentally friendly as there is no harmful by-product in its production. process is very environmentally friendly as there is no harmful by-product in its production. The The performance of BF under high and low temperature is satisfactory, and it also presents promising performance of BF under high and low temperature is satisfactory, and it also presents promising properties regarding stability and thermal insulation. In this paper, the 6 mm chopped basalt fiber properties regarding stability and thermal insulation. In this paper, the 6 mm chopped basalt fiber produced by Jiangsu Tianlong Basalt Continuous Fiber Co., Ltd., Yizheng, China, is used in the produced by Jiangsu Tianlong Basalt Continuous Fiber Co., Ltd., Yizheng, China, is used in the experiment. The macro image of basalt fiber is shown in Figure1. It can be seen from the macro image experiment. The macro image of basalt fiber is shown in Figure 1. It can be seen from the macro image that the color of basalt fiber is golden brown, and it separates from each other. that the color of basalt fiber is golden brown, and it separates from each other.

Figure 1. Macro image of basalt fiber fiber.. Lignin fiber (LF) is made from natural wood. Various application requirements can be obtained Lignin fiber (LF) is made from natural wood. Various application requirements can be obtained via a series of chemical treatments to LF [20], during which the functional groups on the molecular via a series of chemical treatments to LF [20], during which the functional groups on the molecular structure of lignin changed. LF is flocculent, it tends to agglomerate and absorb moisture, so it is not structure of lignin changed. LF is flocculent, it tends to agglomerate and absorb moisture, so it is not suitablesuitable to to be be stacked stacked for for a a long long time time in in spite spite of of its its good good chemical chemical stability. stability. The The LF LF used used in in this this paper paper is producedis produced by by The The JRS JRS company company of Germany,of Germany Rosenberg,, Rosenberg Germany., Germany The. The macro macro image image of of it it is is shown shown in Figurein Figure2. It 2 can. It becan seen be seen from from the macrothe macro image image that thethat micro the micro morphology morphology of LF of is LF quite is quite di fferent different from thatfrom of that BF. LFof BF. is flocculent LF is flocculent and it is and similar it is similar to cotton. to cotton. Its color Its is color gray andis gray it is and easy it tois beeasy stuck to be together. stuck together.

Figure 2. Macro image of lignin fiber ﬁber..

TheThe properties properties of of basalt basalt and and lignin lignin fibers fibers given given by by the the manufacturers manufacturers are are shown shown in in Table Table2. The 2. The PH valuePH value was was tested tested by the by aqueousthe aqueous extract extract method, method, and and fibers fibers were were added added into into the the aqueous aqueous solution solution to maketo make the the test test samples. sample Its.can It can be be concluded concluded from from Table Table2 that 2 that the the pH pH value value and and specific specific surface surface area area of LFof are LF larger are larger than those than thoseof BF; ofthe BF; length–diameter the length–diameter ratio (LDR), ratio heat (LDR), resistance, heat resistance and fracture, and strength fracture of BFstrength are larger of BF than are those larger of than LF. The those increase of LF.of The the increase viscosity of of the fiber-reinforced viscosity of fiber asphalt-reinforced depends asphalt on the depends on the content of fiber and the Einstein coefficient KE [21]. The Einstein coefficient KE is positively correlated with the length–diameter ratio and the viscosity of asphalt. Therefore, BF can better improve the viscosity of asphalt. The PH value of LF is larger than BF and they are all alkaline,

Materials 2020, 13, 2520 4 of 17

content of fiber and the Einstein coefficient KE [21]. The Einstein coefficient KE is positively correlated with the length–diameter ratio and the viscosity of asphalt. Therefore, BF can better improve the Materials 2020, 13, x FOR PEER REVIEW 4 of 17 viscosity of asphalt. The PH value of LF is larger than BF and they are all alkaline, which means that LFwhich has a better means chemical that LF bondinghas a better with chemical asphalt because bonding asphalt with asphalt is a weak because acid materialasphalt is [22 a]. weak It is clearacid thatmaterial these two [22]. fibers It is clear have that their these own two advantages, fibers have which their could own advantages further verify, which the feasibility could further of adding verify difftheerent feasibility fibers into of adding asphalt different to get a fibers better into performance asphalt to of get asphalt a better mortar. performance of asphalt mortar.

TableTable 2. 2.Properties Properties of of basalt basalt ﬁber fiber and and lignin lignin ﬁber. fiber.

IndexIndex BasaltBasalt Fiber Fiber LigninLignin Fiber Fiber Length, mmmm 66 0.80.8 (Average) (Average) Diameter,Diameter, µ μmm 1414 8 8 LengthLength–diameter–diameter ratio ratio 428.6428.6 100100 3 Density, gg/cm/cm 3 2.7102.710 0.9100.910 Speciﬁc surface area, m2/g 0.15 1.93 Specific surface area, m2/g 0.15 1.93 Hygroscopic rate, % 1.63 28.70 HygroscopicHeat resistance, rate, °C % 15501.63 26028.70 Heat PHresistance, value ℃ 7.11550 7.6 260 FracturePH strength, value MPa 20007.1 <3007.6 ≥ ModulusFracture of strength, elasticity, MPa GPa 100≥2000 30＜300 Modulus of elasticity, GPa 100 30 2.2. Experimental Design 2.2. Experimental Design The ﬂowchart of the experiments in this study is shown in Figure3. The flowchart of the experiments in this study is shown in Figure 3.

Tests of raw materials (Asphalt Basalt Fiber Lignin Fiber)

Decision of fiber dosage Decision of fiber-mix-ratio

FMRA sample preparation

Physical properties tests SEM tests Rheological properties tests

High temperature property: Analysis of the Complex modulus Softening point strengthening Phase angle Low temperature property: Ductility mechanism Rutting factor

Asphalt absorption rate tests

Comprehensive performance evaluation analysis using efficacy coefficient method

Figure 3. Flowchart of the experiments. Figure 3. Flowchart of the experiments. 2.3. Test Methods 2.3. Test Methods 2.3.1. Selection of Fiber Mix Ratio and Fiber Content 2.3.1. Selection of Fiber Mix Ratio and Fiber Content Fan et al. [23] conducted research on the performance of asphalt mixture reinforced by basalt fiber, and theyFan found et al. that[23] theconducted optimum research content on of the fiber performance in the basalt of fiber-reinforced asphalt mixture asphalt reinforced mixture by basalt was 0.3%fiber, of theandweight they found of asphalt that the mixture. optimum However, content inof thefiber study in the about basalt fiber-reinforced fiber-reinforced asphalt, asphalt the mixture fiber contentwas 0.3% should of the be weight expanded of asphalt to ensure mixture. a more However, credible in result. the study Zhang about Min fiber [24]- tookreinforced 3% of asphalt, the weight the fiber content should be expanded to ensure a more credible result. Zhang Min [24] took 3% of the weight asphalt as the recommended content of LF in the study of fiber-reinforced asphalt. Therefore, in this paper, 3% is selected as the total content of BF and LF in the SBS-modified asphalt to make different mixed fiber-reinforced asphalt (MFRA), and the different fiber mix ratios shown in Table 3

Materials 2020, 13, 2520 5 of 17 asphalt as the recommended content of LF in the study of fiber-reinforced asphalt. Therefore, in this paper, 3% is selected as the total content of BF and LF in the SBS-modified asphalt to make different mixedMaterials fiber-reinforced 2020, 13, x FOR PEER asphalt REVIEW (MFRA), and the different fiber mix ratios shown in Table3 were5 set of 17 to study the influence of fiber mix ratio (FMR) on the performance of MFRA. FMR value means “Content were set to study the influence of fiber mix ratio (FMR) on the performance of MFRA. FMR value of BF:Content of LF”. means “Content of BF:Content of LF”. Table 3. Fiber mix ratio. BF: basalt fiber, LF: lignin fiber. Table 3. Fiber mix ratio. BF: basalt fiber, LF: lignin fiber. BF:LF 3:0 2:1 1.5:1.5 1:2 0:3 BF:LF 3:0 2:1 1.5:1.5 1:2 0:3

2.3.2. Physical Properties Tests 2.3.2. Physical Properties Tests SofteningSoftening pointpoint waswas chosenchosen toto representrepresent the high-temperaturehigh-temperature property property of of MFRA, MFRA, and and ductility ductility (5(5◦ C,°C , 5 5 cm cm/min)/min) waswas chosenchosen toto representrepresent thethe low-temperaturelow-temperature property. The The tests tests were were conducted conducted accordingaccording toto ASTMASTM D36D36 [[2525]] andand ASTMASTM D113 [26],], and all the tests are are conducted conducted on on three three parallel parallel samples.samples. IfIf thethe errorerror betweenbetween thethe valuevalue of each test and the average average value value is is within within 10%, 10%, the the average average valuevalue is is taken taken as as the the final finalresult. result. IfIf not,not, allall thethe teststests will be conducted again. 2.3.3. Rheological Properties Tests 2.3.3. Rheological Properties Tests The dynamic shear rheometer used in this paper is produced by Malvern Instruments Co., Ltd., The dynamic shear rheometer used in this paper is produced by Malvern Instruments Co., Ltd, Malvern, UK, as is shown in Figure4. According to ASTM D7175 [ 27], the diameters of the parallel Malvern, UK, as is shown in Figure 4. According to ASTM D7175 [27], the diameters of the parallel plates (Figure5) are 25 mm, and the gap value is set as 1000 µm. The strain-controlled loading plates (Figure 5) are 25 mm, and the gap value is set as 1000 μm. The strain-controlled loading mode mode is adopted in this paper, and the controlled strain was set as 1%. The test samples were placed is adopted in this paper, and the controlled strain was set as 1%. The test samples were placed betweenbetween these these twotwo parallelparallel plates.plates. During the test, the lower plate plate remains remains fixed fixed and and the the upper upper plate plate continuouslycontinuously swingsswings backback andand forthforth aroundaround the central axis. It It is is obvious obvious that that the the driving driving loads loads on on asphaltasphalt pavement pavement isis dynamic,dynamic, soso thethe dynamicdynamic shear frequency is is closely closely related related to to the the traffic traffic volume volume ofof the the pavement. pavement. ForFor example,example, high frequency can can simulate simulate heavy heavy tr traafficffic or or high high speed speed,, and and low low frequencyfrequency cancan simulatesimulate lightlight tratraffic.ffic. Therefore, in order to to make make the the tests tests more more similar similar to to the the actual actual situation,situation, the the complex complex modulus modulus (G (G*),*), phase phase angle angle (δ), (δ) and, and rutting rutting factor factor (G*/ sin(G*/sinδ) of δ) MFRA of MFRA were testedwere attested 60 ◦C at with 60 °C di ffwitherent differ testent frequencies. test frequencies.

FigureFigure 4. Dynamic shear rheometer rheometer..

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Figure 5. The parallel plates. Figure 5. The parallel plates.. 2.3.4. Oil (Asphalt) Absorption Rate Tests 2.3.4. Oil (Asphalt) Absorption Rate Tests The surface of the fiber and the fiber itself can absorb a certain amount of asphalt binder The surface of the fiber and the fiber itself can absorb a certain amount of asphalt binder in the in theThe asphalt surface mixture. of the fiber The and oil absorptionthe fiber itself rate can can absorb affect a thecertain properties amount ofof asphaltasphalt mixturesbinder in the [28 ]. asphalt mixture. The oil absorption rate can affect the properties of asphalt mixtures [28]. Within Withinasphalt some mixture. range, The the higheroil absorption the absorption rate can rate, affect the lessthe likelyproperties the fiber-reinforced of asphalt mixtures asphalt [28 mixture]. Within will some range, the higher the absorption rate, the less likely the fiber-reinforced asphalt mixture will susomeffer fromrange, pavement the higher distress the absorption such as oil-bleeding rate, the less or rutslikely at the high fiber temperatures.-reinforced Therefore,asphalt mixture the asphalt will suffer from pavement distress such as oil-bleeding or ruts at high temperatures. Therefore, the absorptionsuffer from capacity pavement of fibers distress is an such important as oil-bleeding index to evaluate or ruts at the high performance temperatu ofres. asphalt Therefore, mortar the or asphalt absorption capacity of fibers isis anan importantimportant indexindex toto evaluateevaluate thethe performanceperformance ofof asphaltasphalt mixture. In this paper, the oil absorption rate of basalt fiber (BF) and lignin fiber (LF) is tested by basket mortar or mixture. In this paper, the oil absorption rate of basalt fiber (BF) and lignin fiber (LF) is leak test. The basket used in the test is shown in Figure6; the diameter of the basket leaking hole is tested by basket leak test. The basket used in the test is shown in Figure 6;; the diameter of the basket 0.315 mm, and the weight of the basket is 200 grams. leakingleaking holehole isis 0.3150.315 mm,, and the weight of the basket is 200 grams.

Figure 6. Basket used in the tests. Figure 6. Basket used in the tests.. The steps of the test are as follows: (1) Place BF and LF separately into two cups, and put it in an The steps of the test are as follows: (1) Place BF and LF separately into two cups, and put it in an oven (105 ◦C 5 ◦C) for 2 h. (2) Take BF and LF out according to the fiber mix ratio (FMR), making sure oven (105 °C± ± 5 °C) for 2 h. (2) Take BF and LF out according to the fiber mix ratio (FMR), making the total weight (m1) is 9 grams, and put it in a beaker. (3) Put the beaker on the tray balance and sure the total weight (m11) is 9 grams, and put it in a beaker. (3) Put the beaker on the tray balance and press the reset button. (4) Pour 300 grams of hot SBS asphalt into the beaker and stir it evenly with a press the reset button. (4) Pour 300 grams of hot SBS asphalt into the beaker and stir it evenly with a glass rod. (5) Measure the weight of the clean basket and mark it as m2. (6) Put the basket on the top of glass rod. (5) Measure the weight of the clean basket and mark it as m22.. (6) Put the basket on the top a collection container and pour the fiber–asphalt mixture all into the baskets and be careful not to make of a collection container and pour the fiber–asphalt mixture all into the baskets and be careful not to the test sample drip or drain in the testing process (the diameter of the collection container is bigger make the test sample drip or drain in the testing process (the diameter of the collection container is than that of the basket). (7) Put the basket along with the collection container into an oven (165 C) bigger than that of the basket).. (7) Put the basket along with the collection container into an oven◦ for 12 h until the weight of the basket is constant. (8) Measure the weight of the basket (with fibers (165 °C ) for 12 h until the weight of the basket is constant. (8) Measure the weight of the basket (with and asphalt in it) and mark it as m3. The asphalt (oil) absorption rate (OA) is calculated according to fibers and asphalt in it) and mark it as m33.. The The a asphalt (oil) absorption rate (OA) is calculated Equationaccording (1). to Equation (1).. m3 m2 m1 = 푚3 − 푚2 − 푚1 OA푂퐴 = 3 − 2 − 1 (1) 푂퐴 = m푚1 (1) 푚1

2.3.5. SEM Tests

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2.3.5.Materials SEM 2020 Tests, 13, x FOR PEER REVIEW 7 of 17

ScanningScanning electron electron microscope microscope (SEM) used in this paper is is produced produced by by Philips Philips company company Amsterdam,Amsterdam, thethe Netherlands. Netherlands. Micro Micro images images of of different different kinds kinds of of mixed mixed fiber fiber-reinforced-reinforced asphalt asphalt (MFRA)(MFRA) are are taken taken in in aa vacuumvacuum condition,condition, and the fiber fiber mix ratios ratios (FMR (FMRs)s) adopted adopted in in this this test test are are 3:0, 3:0, 1.5:1.5,1.5:1.5, and and 0:3 0:3 becausebecause whenwhen thethe FMRFMR isis 2:1 or 1:2, the micro morphology is is almost almost the the same same as as the the micromicro morphology morphology whenwhen thethe FMRFMR isis 1.5:1.5.1.5:1.5. The purpose of of this this tes testt is is to to explain explain the the strengthening strengthening mechanismmechanism of of thethe fibersfibers whenwhen theythey areare combiningcombining withwith the asphalt. TheThe stepssteps ofof the sample preparation preparation are are as as follows: follows: (1) (1) Prepare Prepare the theMFRA MFRA sample sample in a inmold a mold.. (2) (2)Put Put it itin ina afreezer freezer and and freeze freeze it itfor for 3 3h. h. (3 (3)) Slice Slice the the sample sample with with a aclean clean blade. blade. (4 (4)) Dry Dry it it in in room room temperaturetemperature andand makemake suresure therethere isis nono waterwater onon thethe surface.surface. ( (5)5) Plate Plate gold gold in in the the vacuum vacuum coating coating machinemachine (Figure (Figure7 ).7). Then, Then, the the sample sample preparation preparation isis completed.completed.

Figure 7. SEM test samples. Figure 7. SEM test samples. 2.3.6. Comprehensive Evaluation Method 2.3.6. Comprehensive Evaluation Method The efficacy coefficient method (ECM) has been used by many researchers to evaluate their subjectsThe [29 e,fficacy30]. In coefficientthis section, method ECM was (ECM) used has to comprehensively been used by many evaluate researchers the performance to evaluate of mixed their fiber-reinforcedsubjects [29,30].asphalt In this section, under diECMfferent was fiber used mix to comprehensively ratios. According evaluate to the principle the performance of multi-objective of mixed programming,fiber-reinforced the asphalt efficacy under coe differentfficient method fiber mix requires ratios. According selecting atosatisfactory the principle value of multi as- theobjective upper limitprogramming, and a disallowed the efficacy value coefficient as the lower method limit requires to calculate select theing a average satisfactory efficacy value coe asffi thecient upper value limit (F), usingand a Equations disallowed (2) value and (3). as the A bigger lower average limit to ecalculatefficacy coe theffi cientaverage value efficacy (F) means coefficient a better value comprehensive (F), using Equations (2) and (3). A bigger average efficacy coefficient value (F) means a better comprehensive performance of MFRA. In Equations (2) and (3), fi is the efficacy coefficient value of a single index; performance of MFRA. In Equations (2) and s(3), fi is the efficacy coefficient value of a single index;h xi is the actual test value of a single index; x is the disallowed value of a single index; and x is the i s i x h satisfactoryxi is the actual value test of avalue single of index. a single index; i is the disallowed value of a single index; and xi is s the satisfactory value of a single index. xi xi fi = − (2) xh xs ixx− i s f  ii i nh s (2) Pxxi  i fi i=1 F = n (3) n fi F  i=1 (3) 3. Results and Discussion n 3.1. Softening Point 3. Results and Discussion It can be seen from Figure8 that the fiber mix ratio (FMR) has a great influence on the increase of the3.1. softening Softening point.Point By comparison and calculation, the softening point of asphalt with 3% BF (FMR is 3:0) increased by 12.9% compared with that without fiber. In the previous analysis of the macroscopic It can be seen from Figure 8 that the fiber mix ratio (FMR) has a great influence on the increase and microscopic morphology of BF, it is clear that BF is not easy to agglomerate. Therefore, BF can of the softening point. By comparison and calculation, the softening point of asphalt with 3% BF (FMR disperse more evenly in the SBS asphalt and form a three-dimensional network structure [28,31], is 3:0) increased by 12.9% compared with that without fiber. In the previous analysis of the which could make the MFRA more stable and hinder the flow of asphalt to some extent and eventually macroscopic and microscopic morphology of BF, it is clear that BF is not easy to agglomerate. makeTherefore the softening, BF can disperse point higher. more evenly in the SBS asphalt and form a three-dimensional network structure [28,31], which could make the MFRA more stable and hinder the flow of asphalt to some extent and eventually make the softening point higher.

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In the previous comparison of BF and LF properties, it shows that the heat resistance of BF is InIn the the previous previous comparison comparison ofof BFBF andand LFLF properties,properties, itit showsshows that the heat resistance of of BF BF is is nearly 5 times higher than that of LF. With the increase of temperature, the property of LF in the nearlynearly 5 times5 times higher higher than than that that of LF.of WithLF. With the increasethe increase of temperature, of temperature, the property the property of LF inof the LF asphaltin the asphalt is more unstable, which could lead to the decrease of the enhancement effect of fibers on the isasphalt more unstable, is more unstable, which could which lead could to the lead decrease to the ofdecrease the enhancement of the enhancement effect of fibers effect onof thefibers softening on the softening point. The softening point of asphalt with 3% LF (FMR is 0:3) increased only by 0.5% point.softening The softening point. The point softening of asphalt point with of asphalt 3% LF (FMR with is3% 0:3) LF increased (FMR is only0:3) increased by 0.5% compared only by 0.5% with compared with that without fiber. The lower the FMR value, the worse the enhancement effect on the thatcompared without with fiber. that The without lower fiber. the The FMR lower value, the the FMR worse value, the the enhancement worse the enhancement effect on the effect softening on the softening point will be. It can be concluded that BF can better improve the high-temperature pointsoften willingbe. point It can will be be. concluded It can be that concluded BF can better that improve BF can better the high-temperature improve the high performance-temperature of performance of SBS asphalt. SBSperformance asphalt. of SBS asphalt.

92 90.3 92 90.3 90 90 87.8 88 87.8 88 86 86 84 83.2 84 83.2 82 81.1 82 80 81.1 80.4 80 80 80.4 80

Softening point/℃Softening 78

Softening point/℃Softening 78 76 76 74 74 0:0 3:0 2:1 1.5:1.5 1:2 0:3 0:0 3:0 2:1 1.5:1.5 1:2 0:3 Content of BF:Content of LF Content of BF:Content of LF Figure 8. Test results of the softening point. Figure 8. Test results of the softening point. 3.2. Ductility 3.2. Ductility According to Figure9, 3% basalt fiber can significantly improve the low-temperature ductility According to Figure 9, 3% basalt fiber can significantly improve the low-temperature ductility of SBS-modifiedAccording asphalt.to Figure After 9, 3% calculation, basalt fiber thecan ductility significantly of it increasedimprove the by low 11.1%-temperature compared ductility with the of SBS-modified asphalt. After calculation, the ductility of it increased by 11.1% compared with the asphaltof SBS without-modified fiber. asphalt The. resultsAfter calculation, are in accordance the ductility with the of testit increased results of by softening 11.1% compared point. As with is shown the asphalt without fiber. The results are in accordance with the test results of softening point. As is inasphalt Table2 ,without the fracture fiber. strength The results and the are elasticity in accordance modulus with of BFthe is test much results better of than softening LF. Then, point. BF couldAs is shown in Table 2, the fracture strength and the elasticity modulus of BF is much better than LF. Then, participateshown in Table more 2, in the the fract testure process strength than and LF the and elasticity disperse modulus some part of BF of theis much stress better [28], than and thisLF. Then could, BF could participate more in the test process than LF and disperse some part of the stress [28], and leadBF could to the participate increase of more ductility. in the This test is process also in than accordance LF and withdisperse the findingssome part that of BFthe canstress enhance [28], and the this could lead to the increase of ductility. This is also in accordance with the findings that BF can stithisffness could capacity lead to of the asphalt increase [31]. of ductility. This is also in accordance with the findings that BF can enhance the stiffness capacity of asphalt [31]. 45 42.1 45 42.1 38.7 40 37.9 38.7 36.9 35.4 40 37.9 36.9 35.4 35 35 30 30 25 23.7 25 23.7 20 20

Ductility/cm 15

Ductility/cm 15 10 10 5 5 0 0 0:0 3:0 2:1 1.5:1.5 1:2 0:3 0:0 3:0 2:1 1.5:1.5 1:2 0:3 Content of BF:Content of LF Content of BF:Content of LF Figure 9. Test results of ductility. Figure 9. Test results of ductility. It can be seen that when the fibers are all LF (FMR is 0:3), the ductility of MFRA is even lower It can be seen that when the fibers are all LF (FMR is 0:3), the ductility of MFRA is even lower than thatIt can of SBSbe seen asphalt that withoutwhen the fiber, fibers and are the all ductilityLF (FMR decreasedis 0:3), the byductility 37.5%. of This MFRA phenomenon is even lower can than that of SBS asphalt without fiber, and the ductility decreased by 37.5%. This phenomenon can bethan explained that of bySBS three asphalt reasons. without Firstly, fiber the, and length the ductility of LF is onlydecreased 13.3% by of 37.5%. the length This of phenomenon BF, which could can be explained by three reasons. Firstly, the length of LF is only 13.3% of the length of BF, which could makebe explained the fibers by harder three reasons. to transmit Firstly, the the stress. length Secondly, of LF is theonly fracture 13.3% of strength the length of LFof BF is much, which smaller could make the fibers harder to transmit the stress. Secondly, the fracture strength of LF is much smaller thanmake BF. the The fibers total harder content to of transmit fibers is the set stress. as 3%, Secondly, so when thethe contentfracture of strength LF is bigger, of LF theis much content smaller of BF than BF. The total content of fibers is set as 3%, so when the content of LF is bigger, the content of BF willthan be BF. smaller. The total This content will make of fibers the fibersis set as bear 3%, less so when stress the in thecontent MFRA of LF because is bigger, LF couldthe content not bear of BF as will be smaller. This will make the fibers bear less stress in the MFRA because LF could not bear as muchwill be stress smaller. as BF This does. will Thirdly, make LFthe isfibers more bear likely less to stress agglomerate in the MFRA in asphalt, because which LF could is consistent not bear with as much stress as BF does. Thirdly, LF is more likely to agglomerate in asphalt, which is consistent with themuch macro stress morphology as BF does. of Thirdly, it, so adding LF is more LF into likely the to asphalt agglomerate might in make asphal thet, propertywhich is consistent of MFRA morewith the macro morphology of it, so adding LF into the asphalt might make the property of MFRA more unstablethe macro and morphology eventually makeof it, so the adding ductility LF decrease.into the asphalt might make the property of MFRA more unstable and eventually make the ductility decrease.

Materials 2020, 13, 2520 9 of 17 Materials 2020, 13, x FOR PEER REVIEW 9 of 17

Materials 2020, 13, x FOR PEER REVIEW 9 of 17 3.3. Rheological Properties 3.3. Rheological Properties 3.3. RheologicalThe complex Properties modulus (G*) can be divided into two parts: elasticity and viscosity. A larger G* The complex modulus (G*) can be divided into two parts: elasticity and viscosity. A larger G* value represents the greater stiffness and the deformation resistance of asphalt. Phase angle (δ) can value representsThe complex the modulus greater sti (G*)ffness can and be divided the deformation into two resistanceparts: elasticity of asphalt. and viscosity. Phase angle A larger (δ) can G* stand for the time lag between the stress and the strain in the test. A lower phase angle means better standvalue for represents the time lag the between greater st theiffness stress and and the the deformation strain in the resistance test. A lower of asphalt. phase anglePhase meansangle (δ) better can elasticity. The rutting factor (G*/sin δ) can be used to access the high-temperature stability of asphalt, elasticity.stand for The the rutting time lag factor between (G*/ sintheδ stress) can beand used the strain to access in the the test. high-temperature A lower phase stabilityangle means of asphalt, better and a higher rutting factor implies better rutting resistance. The test results of the rheological andelasticity. a higher The rutting rutting factor factor implies (G*/sin better δ) can rutting be used resistance. to access The the test high results-temperatu of the rheologicalre stability propertiesof asphalt, properties of the MFRA under different fiber mix ratios are shown in Figures 10–12. ofand the MFRAa higher under rutting different factor fiber implies mix ratiosbetter are rutting shown resistance. in Figures T 10he– 12 test. results of the rheological properties of the MFRA under50.00 differentSBS, R²=0.9920 fiber mix ratios are3:0, R²=0.9905shown in Figures 10–12. 2:1, R²=0.9976 1.5:1.5, R²=0.9944 50.00 SBS, R²=0.9920 3:0, R²=0.9905 40.00 1:2, R²=0.9944 0:3, R²=0.9917 2:1, R²=0.9976 1.5:1.5, R²=0.9944 40.00 1:2, R²=0.9944 0:3, R²=0.9917 30.00

30.00 G*/kPa 20.00

G*/kPa 20.00 10.00

10.00 0.00 0 5 10 15 0.00 Frequency/Hz 0 5 10 15 Frequency/Hz Figure 10. Test results of complex modulus. Figure 10. Test results of complex modulus. Figure 10. Test results of complex modulus. SBS 3:0 2:1 85 1.5:1.5 1:2 0:3 SBS 3:0 2:1 7585 1.5:1.5 1:2 0:3

6575

° δ/

5565

° δ/ 4555

3545 0 5 10 15 35 Frequency/Hz 0 5 10 15 Figure 11. Test resultsFrequency/Hz of phase angle. Figure 11. Test results of phase angle. It can be seen from Figure 10 Figurethat G 11.* increases Test results after of phase using angle fibers. in the SBS asphalt. With the It can be seen from Figure 10 that G* increases after using fibers in the SBS asphalt. With the increase of the FMR value, the complex modulus becomes lower. This means that LF could better increase of the FMR value, the complex modulus becomes lower. This means that LF could better improveIt thecan stibeff seenness from and theFigure deformation 10 that G* resistance increases of after MFRA. using This fibers phenomenon in the SBS canasphalt. be explained With the improve the stiffness and the deformation resistance of MFRA. This phenomenon can be explained byincrease two aspects. of the Firstly,FMR value, Table the2 shows complex that modulus the pH valuebecomes of LFlower is about. This 7.04%means higherthat LF than could that better of byimprove two aspects the stiffness. Firstly, and Table the 2 deformation shows that the resistance pH value of ofMFRA. LF is aboutThis phenomenon 7.04% higher can than be that explained of BF. BF. As a result of the weak acidity of asphalt, the chemical bond ability between lignin fiber and Asby atwo result aspects of the. Firstly, weak acidity Table 2of shows asphal thatt, the the chemical pH value bond of LF ability is about between 7.04% lignin higher fiber than and that asphalt of BF. asphalt is better than that between basalt fiber and asphalt. Secondly, the texture of LF is softer, isAs better a result than of that the betweenweak acidity basalt of fiber asphal andt, theasphalt chemical. Secondly, bond abilitythe texture between of LF lignin is softer, fiber so and it is asphalt easier so it is easier to blend into asphalt to make its property change and eventually enhance its stiffness tois better blend than into that asphalt between to make basalt its fiber property and asphalt change. Secondly, and eventually the texture enhance of LF is its softer, stiffness so it and is easier the and the deformation resistance. This can be ascribed by the fact that lignin fiber has a lot of polar deformationto blend into resistance. asphalt toThis make can itsbe propertyascribed by change the fact and that eventually lignin fiber enhance has a itslot stiffnessof polar groups and the, groups, similar to the carboxyl groups and the phenolic hydroxyl groups [32], which contribute to similardeformation to the resistance. carboxyl groupsThis can and be ascribed the phenolic by the hydroxyl fact that groupslignin fiber [32], haswhich a lot contribute of polar groups to the , the establishment of interactions with the asphalt molecules and their clusters, and the complex establishmentsimilar to the of carboxyl interactions groups with andthe asphalt the phenolic molecules hydroxyl and their groups clusters [32,] and, which the complex contribute viscosity to the viscosity modulus is determined eventually by the force field between the molecules (the strength modulusestablishment is determined of interactions eventually with the by asphalt the force molecules field between and their the clusters molecules, and the(the complex strength viscosity of the of the interactions involved in the intermolecular network in asphalt) [33]. As is mentioned before, interactionsmodulus is involved determined in the eventually intermolecular by the network force field in asphalt) between [ 33 the]. As molecules is mentioned (the strengthbefore, BF of can the BF can form a three-dimensional network structure and participate more in the softening point and forminteractions a three- dimensionalinvolved in the network intermolecular structure networkand participate in asphalt) more [ 33in ].the As softening is mentioned point before, and ductility BF can ductility test process and disperse some part of the stress [28]. However, the rheology test is to test the testform process a three and-dimensional disperse some network part structure of the stress and [participate28]. However, more the in rheologythe softening test ispoint to test and the ductility inner inner property of the MFRA, so LF shows a better enhancement effect on G*. propertytest process of the and MFRA, disperse so LFsome shows part a of better the stress enhancement [28]. However, effect on the G*. rheology test is to test the inner G propertyItIt can can alsoof also the be MFRA, be concluded concluded so LF thatshows that G** a is better is positively positively enhancement correlated correlated effect with with on G*. the the loading loading frequency. frequency. This This is is becausebecauseIt that canthat asphalt alsoasphalt be isconcludedis aa kind of thatviscoelastic viscoelastic G* is positively material. material. correlatedThe The decrease decrease with of ofthe the loading loading loading time frequency. time will will make This make the is thedeformationbecause deformation that ofasphalt ofasphalt asphalt is asmaller, kind smaller, of viscoelasticwhich which will will eventuallymaterial. eventually The lead lead decrease to to the the ofincrea increase the seloading of of modulus modulus time will in in makethe the test. test. the Besides,deformation FMR ofhas asphalt more of smaller, an impact which on thewill G* eventually when the leadfrequency to the is increa bigger,se meanof modulusing that in when the test.the Besides, FMR has more of an impact on the G* when the frequency is bigger, meaning that when the

Materials 2020, 13, 2520 10 of 17

Materials 2020, 13, x FOR PEER REVIEW 10 of 17 Besides, FMR has more of an impact on the G* when the frequency is bigger, meaning that when thedesigned designed traffic traffi cvolume volume is isbigger, bigger, FMR FMR should should be be paid paid more more attention attention in in the the design of the mixedmixed fiber-reinforcedfiber-reinforced asphalt asphalt mixture. mixture. AsAs is is shown shown in in Figure Figure 11 1,1 the, the phase phase angle angle fluctuates fluctuates with with the the increase increase of of frequency frequency butbut thethe fluctuationfluctuation range range is is limited, limited, which whichis ismainly mainly because because of of the the significant significant delayed effecteffect of MFRA. Furthermore,Furthermore, the the fluctuation fluctuation phenomenon phenomenon is is in in accordance accordance with with the the test test results results given by Gu [ 4].]. WhenWhen the the FMR FMR is is 2:1, 2:1, 0:3, 0:3, 1.5:1.5, 1.5:1.5, 1:2, 1:2, and and 3:0, 3:0, the the phase phase angle angle of of MFRA MFRA decreases decreases inin turn.turn. WhenWhen thethe fibersfibers are are all all BF BF (FMR (FMR is is 3:0), 3:0), the the phase phase angle angle is is the the smallest, smallest, which whichmeans means thatthat thethe elasticelastic propertyproperty is thethe best. best. This This is is mainly mainly due due to to the the high high elastic elastic modulus modulus of of BF. BF. According According toto thethe mechanicalmechanical principleprinciple ofof composite composite materials, materials, the the overall overall elasticity elasticity composite composite materialmaterial isis decideddecided byby thethe elasticityelasticity of its composition.composition. So, So, the the high high elastic elastic modulus modulus of of BF BF will willdefinitely definitely increaseincrease thethe elasticelastic modulus of MFRA.

50.00 SBS, R²=0.9897 3:0, R²=0.9855 2:1, R²=0.9953 1.5:1.5, R²=0.9893 40.00 1:2, R²=0.9931 0:3, R²=0.9905

kPa 30.00 δ/

20.00 G*/sin

10.00

0.00 0 5 10 15 Frequency/Hz Figure 12. Test results of rutting factor. Figure 12. Test results of rutting factor. From Figure 12, the change pattern of G*/sin δ is the same as G*. LF shows a better enhancement From Figure 12, the change pattern of G*/sin δ is the same as G*. LF shows a better enhancement effect on G*/sin δ than BF, and FMR has more of an impact on the G*/sin δ when the frequency is bigger. effect on G*/sin δ than BF, and FMR has more of an impact on the G*/sin δ when the frequency is The reasons for these conclusions are the same as that of G*. bigger. The reasons for these conclusions are the same as that of G*. 3.4. Oil Absorption Rate of Mixed Fibers 3.4. Oil Absorption Rate of Mixed Fibers Table4 presents that when the fibers are all LF (FMR is 0:3), the oil absorption rate is the biggest, and theTable amount 4 presents of asphalt that that when LF the could fibers absorb are all 8.4 LF times (FMR its is own 0:3), weight. the oil absorption This is the reasonrate is the why biggest lignin, fiberand is the widely amount used of in asphalt the SMA that asphalt LF could mixtures absorb to8.4 absorb times asphalt.its own weight. With the This increase is the of reason BF, the why oil lignin fiber is widely used in the SMA asphalt mixtures to absorb asphalt. With the increase of BF, absorption rate gradually gets smaller, and when the fibers are all BF (FMR is 3:0), the oil absorption the oil absorption rate gradually gets smaller, and when the fibers are all BF (FMR is 3:0), the oil rate is the smallest: just half of that of LF. This result means that the oil absorption ability of LF is much absorption rate is the smallest: just half of that of LF. This result means that the oil absorption ability better than that of BF [28]. The specific surface area of LF is 12.87 times of that of BF. Under the same of LF is much better than that of BF [28]. The specific surface area of LF is 12.87 times of that of BF. evaluation standard (the same weight), LF will have more contact area with asphalt, which will make Under the same evaluation standard (the same weight), LF will have more contact area with asphalt, its oil absorption rate higher. The micro image of LF shows that lignin fibers can interweave with which will make its oil absorption rate higher. The micro image of LF shows that lignin fibers can each other, which will form a mechanical riveting structure to hold more asphalt. All these reasons interweave with each other, which will form a mechanical riveting structure to hold more asphalt. contribute to the higher oil absorption rate of LF. All these reasons contribute to the higher oil absorption rate of LF.

Table 4. Results of the basket leak tests. OA: (oil) absorption rate. Table 4. Results of the basket leak tests. OA: (oil) absorption rate. BF:CF 3:0 2:1 1.5:1.5 1:2 0:3 BF:CF 3:0 2:1 1.5:1.5 1:2 0:3 m1,g9999999999m1, g 9 9 9 9 9 9 9 9 9 9 m2, g 199.8 200 199.6 200 200 201 200.2 200 200 199.9 m2, g 199.8 200 199.6 200 200 201 200.2 200 200 199.9 m3, g 247.5 245.9 259.9 260.3 265.7 270.3 274 275.6 282.8 286.3 OA m3, g4.3 247.5 4.1 245.9 5.7 259.9 5.7260.3 6.3265.7 6.7270.3 7.2274 275.6 7.4 282.8 8.2 286.3 8.6 AverageOA 4.24.3 4.1 5.7 5.7 6.3 6.5 6.7 7.2 7.37.4 8.2 8.48.6 Average 4.2 5.7 6.5 7.3 8.4

3.5. Comprehensive Performance Evaluation In the comprehensive evaluation of MFRA using the efficacy coefficient method (ECM) [29–30], the adopted indexes in the calculation of efficacy coefficient value are the softening point, ductility,

Materials 2020, 13, 2520 11 of 17

3.5. Comprehensive Performance Evaluation In the comprehensive evaluation of MFRA using the efficacy coefficient method (ECM) [29,30], the adopted indexes in the calculation of efficacy coefficient value are the softening point, ductility, rutting factor, and oil absorption rate. The softening point represents the high-temperature performance, ductility represents the low temperature performance, the rutting factor represents the rheological properties, and the oil absorption rate represents the asphalt absorption ability of the mixed fibers. In the calculation, the disallowed value is set as the worst test value of the test value vector of a single index (such as the softening point) under different fiber mix ratios, and the satisfactory value is set as the best test value of the test value vector of a single index. It should be mentioned that the test value vector of the rutting factor is the average test value vector under all the load frequencies. The result in Table5 shows that when the FMR is 1:2, the comprehensive performance of MFRA is the best. However, in the design of a specific fiber-reinforced asphalt having a specific performance priority, it is necessary to determine the FMR according to the test results of that specific index (such as softening point).

Table 5. Calculation of the eﬃcacy coeﬃcient.

BF:LF Index Satisfactory Value Disallowed Value 3:0 2:1 1.5:1.5 1:2 0:3 90.3 87.9 83.2 81.1 78.4 Softening point/ C 90.3 78.4 ◦ 1.0000 0.7983 0.4034 0.2269 0.0000 42.1 38.7 36.9 35.4 23.7 Ductility/cm 42.1 23.9 1.0000 0.8152 0.7174 0.6359 0.0000 17.07 18.61 20.56 22.50 25.58 Rutting factor/kPa 25.58 17.07 0.0000 0.1812 0.4106 0.6385 1.0000 4.2 5.7 6.5 7.3 8.4 Oil absorption rate 8.4 4.2 0.0000 0.3571 0.5476 0.7381 1.0000 Average eﬃcacy coeﬃcient value 0.5000 0.5380 0.5197 0.5598 0.5000

3.6. Micromorphology of Fibers and Test Samples A variety of composite interface theories have been put forward in studies about composite materials, and each theory has a certain experimental basis and can explain some experimental phenomena. However, due to the complexity of the mechanism of composite interface, the research studies on it are still very superficial. Therefore, in this section, scanning electron microscopy (SEM) was used to observe the micro morphology and interface structure of fiber-reinforced asphalt, and chemical bond theory, interface layer theory, friction theory, etc. [34] are adopted to analyze the strengthening mechanism of SBS asphalt reinforced by mixed fibers under different fiber mixing ratios (FMRs).

3.6.1. Micro Morphology of Fibers The micro images of basalt fibers are shown in Figure 13. From the micro image, the shape of a single basalt fiber is similar to that of a cylinder, and it has a smooth surface. The diameter and the thickness are very uniform, which will contribute to the stability of BF. There are some little bulges on the surface of basalt fiber, which is mainly caused by the high temperature [19] during its production. The micro images of lignin fibers are shown in Figure 14. It can be seen that lignin fibers interweave with each other, which will make it hard to disperse. The shape of LF is similar to that of a curve, and the surface of it is very rough. Materials 2020, 13, 2520 12 of 17 Materials 2020, 13, x FOR PEER REVIEW 12 of 17

(a) (b)

FigureFigure 13.13. MicroMicro imageimage ofof BF:BF: ((aa)) 6060×;;( (b) 800 ×. . × ×

(a) (b)

FigureFigure 14.14. MicroMicro imageimage ofof LF:LF: ((aa)) 6060×;;( (b) 800 ×. . × × 3.6.2.3.6.2. Micromorphology Micromorphology ofof MFRAMFRA SamplesSamples SEMSEM was was usedused toto observeobserve thethe micromicro morphologiesmorphologies of mixed fiber fiber-reinforced-reinforced asphalt asphaltss (MFRA) (MFRA) withwith three three di differentfferent fiber fiber mix mix ratios ratios (FMRs): (FMRs): 3:0,3:0, 1.5:1.51.5:1.5 andand 0:3. The The micro micro images images of of these these three three kinds kinds ofof MFRAs MFRA ares are shown shown in Figures in Figure 15s– 1715.– According17. According to the to SEM the images SEM images and composite and composite interface interface theories, ittheories, shows that it shows there that is an there interface is an layerinterface between layer thebetween fiber andthe fiber asphalt, and andasphalt this, and could this make could the make fiber bondthe fiber with bond the asphalt with the better. asphalt The better. property The ofproperty the interface of the layerinterface is decided layer is by decided the properties by the properties of asphalt andof asphalt fiber, and and it will fiber, eventually and it will aff eventuallyect the performance affect the of performance the fiber–asphalt of the composite fiber–asphalt material. composite material.Table 2 shows that the pH value of basalt fiber is 7.1, and this means that BF is alkaline. Thus,Table BF could 2 shows bond that with th asphalte pH value with theof basalt chemical fiber bonds is 7.1 because, and this asphalt means is that a weak BF is acid alkaline. material Thus, [22 ]. TheBF previouscould bond section with showsasphalt that with there the are chemical also some bonds little because bulges asphalt on the surfaceis a weak of basaltacid material fiber, and [22 this]. willThe increase previous the section friction shows between that there BF and are asphalt. also some It can little be bulges seen from on the Figure surface 15a o thatf basalt when fiber bonding, and withthis asphalt, will increase BF is completelythe friction wrapped between BF by theand asphalt, asphalt.and It can the be interface seen from layer Figure also has 15a some that bulges. when Accordingbonding with to Figure asphalt, 15b, BF there is completely are many wrapped basalt fibers by exposedthe asphalt on, theand fracture the interface surface layer of thealso test has sample, some whichbulges. could According contribute to Figure to the 1 enhancement5b, there are many of the basalt performance fibers exposed of the asphalt. on the fracture surface of the test sample, which could contribute to the enhancement of the performance of the asphalt.

Materials 2020, 13, 2520 13 of 17 Materials 2020, 13, x FOR PEER REVIEW 13 of 17 Materials 2020, 13, x FOR PEER REVIEW 13 of 17

(a) (b) (a) (b) FigureFigure 15.15. MicroMicro images images of mixed of mixed-fiber-reinforced-fiber-reinforced asphalt asphalt (MFRA (MFRA)) (3:0): (a (3:0):) image (a 1,) image1000×; ( 1,b) image 1000 ; × (bFigure2,) image1000 ×15.. 2, Micro 1000 images. of mixed-fiber-reinforced asphalt (MFRA) (3:0): (a) image 1, 1000×; (b) image 2, 1000×. × FromFrom Figure Figure 16 16a,a,there there areare aa lotlot ofof foldsfolds onon thethe interfaceinterface layerlayer between LF and asphalt. This This is is mainlymainlyFrom because because Figure that 16 LFa , there is is very very are soft softa lot and and of it folds could it could on curl the curl and interface and contact contact layer with withbetween asphalt, asphalt, whichLF and which will asphalt. absorb will This absorb more is moremainlyasphalt asphalt becauseon the on interface that the interfaceLF is layer. very layer. Asoftccording and According it couldto Figure curl to Figures 1and6b and contact 16 Figb andure with 44,, asphalt, didifferentfferent which shapes will of of absorblignin lignin fibersmore fibers couldasphaltcould interweave interweave on the interface with with eacheach layer. otherother According andand formform to aFigurea mechanicalmechanical 16b and riveting Figure structure, 4, different and and shapes this this structure structureof lignin fiberscould could strengthencouldstrengthen interweave the the properties properties with each of of the otherthe asphalt. asphalt. and form a mechanical riveting structure, and this structure could strengthen the properties of the asphalt.

(a) (b) (a) (b) Figure 16. Micro images of MFRA (0:3): (a) image 1, 800×; (b) image 2, 800×. FigureFigure 16. 16.Micro Micro images images of of MFRA MFRA (0:3):(0:3): ((aa)) imageimage 1,1, 800800×;;( (b)) image image 2, 2, 800 800×. . × × Figure 17 shows that BF and LF scatter irregularly in the asphalt. Basalt fibers and lignin fibers couldFigureFigure interweave 17 17 shows shows with thatthat each BFBF other andand LF LFand scatterscatter lignin irregularlyirregularly fibers could inin wind the asphalt. around Basalt Basalt the basalt fibers fibers fibers, and and lignin ligninwhich fibers fibers will couldcouldform interweavea interweave more solid with withthree eacheach-dimensional otherother andand network ligninlignin fibersfibersstructure. could This wind structure around will the the make basalt basalt the fibers, fibers, bonding which which ability will will formformbetween aa morem orethe solid solidmixed three three-dimensional fibers-dimensional and the asphalt network network better, structure. structure. ensure This the Thisstructure mixed structure fibers will make hold will the makemore bonding asphalt the bonding ability, and abilitybetweeneventually between the improve mixed the mixed fibersthe performance fibersand the and asphalt theof MFRA. asphalt better, Apart better, ensure from ensure the their mixed the respective mixed fibers fibers holdenhancement more hold moreasphalt effects asphalt,, and on andeventuallythe eventuallyasphalt improve matrix, improve tthehis theperformancecombined performance network of MFRA. of MFRA.structure Apart Apart formed from from their by their BFrespective and respective LF enhancementcan enhancementbe classified effects as eff theectson onthenovel the asphalt asphaltor emerging matrix, matrix, properties this this combined combined of the network MFRA network compared structure structure withformed formed the bysingle by BF BF -andfiber and LF reinforced LF can can be be classified classifiedasphalt binder. as as the the novelnovelFurthermore, or or emerging emerging the propertiesfunctionapropertiesl ofgroupsof thethe MFRA of BF compared and LF might with with makethe the single single-fiber this -fiberstructure reinforced reinforced more stableasphalt asphalt, because binder. binder. Furthermore,Furthermore,adding different the the functionaladditives functiona might groupsl groups form of BFof the andBF amphiphilic and LF mightLF might make molecules make this structurethis (from structure the more microscopic stable,more stable because view),, because addingwhich diaddingwillfferent trigger different additives the emerging additives might form propertiesmight the form amphiphilic or the functions amphiphilic molecules [35] .molecules According (from (from the to Figure microscopic the microscopic 17b, BF view), is view),much which longerwhich will triggerwillthan triggerLF the, and emerging the BF emerging is always properties properties straight. or functions This or functions is mainly [35]. [ Accordingbecause35]. According that to the Figure to elasticity Figure 17b, 1 BFmodulus7b, is BF much is mof longeruch BF islonger very than LF,thanhigh and ,LF and BF, and the is always BFtexture is always straight. of BF straight. is very This hard. isThis mainly isThis mainly becausecharacteristic because that the thatwill elasticity themake elasticity the modulus strength modulus ofand BF ofthe is BF veryelasticity is very high, andhighof the the, and MFRA texture the better.texture of BF is of very BF is hard. very Thishard. characteristic This characteristic will make will make the strength the strength and the and elasticity the elasticity of the MFRAof the better.MFRA better.

MaterialsMaterials2020 2020, 13, 1,3 2520, x FOR PEER REVIEW 1414 of of 17 17 Materials 2020, 13, x FOR PEER REVIEW 14 of 17

(a) (b) Figure 17.(a) Micro images of MFRA (1.5:1.5): (a) image 1, 150×; (b) image(b) 2, 300×. FigureFigure 17. 17.Micro Micro images imagesof of MFRAMFRA (1.5:1.5):(1.5:1.5): (a) image 1, 150 ×;;( (bb)) image image 2, 2, 300 300×. . 3.6.3. The Failure Model of Fiber-Reinforced Asphalt × × 3.6.3. The Failure Model of Fiber-Reinforced Asphalt 3.6.3.The The composition Failure Model of fiberof Fiber-reinforced-Reinforced asphalt Asphalt is fiber and asphalt binder. When they mix with each other,TheThe they compositioncomposition will bond ofof togetherfiber-reinforcedfiber-reinforced due to asphalt asphalt the action is fiberfiber of and molecular asphalt forcesbinder binder. and. When When chemical they they mix mix bonds. with with each Theeach other,other,mechanism they they will swill of bond these bond together actions together due are to duevery the to actioncomplex, the of action molecular but ofthey molecular forceswill contribute and forces chemical to and the bonds. chemical strengthening The bonds.mechanisms of Thethe ofmechanismasphalt these actions[36].s Tofhe arethese failure very actions model complex, are of verythe but fiber complex, they-reinforced will but contribute they asphalt will to could contribute the strengtheningbe summarized to the strengthening of into the three asphalt typical of the [36]. Theasphaltsituations failure [36, model].which The failure of are the shown model fiber-reinforced inof the Figure fiber 1asphalt-8reinforced. Figure could 1 asphalt8a be shows summarized could the be failure summarized into situation three typicalinto that three situations,the typical fiber- whichsituationsreinforced are shown, asphaltwhich in are Figureis broken shown 18. due Figure in Figureto the18a slide shows 18. Figureofthe thefailure fiber 18a showsfrom situation the the asphalt thatfailure the, whichfiber-reinforced situation is mainlythat the asphaltbecause fiberis brokenreinforcedthat the due bonding toasphalt the slidebetween is broken of the the fiberdue fiber fromto andthe the slidethe asphalt, asphalt of the which fiberis not from is strong mainly the enough asphalt because ,to which that hold the theis bondingmainly pull-out because between force. thethatThis fiber the mode andbonding can the asphalt be between classified is notthe as strongfiber the andinterface enough the asphalt to failure hold is the mode not pull-out strong [37]. force.Figureenough This 1 8tobmode holdshows the can that pull be theclassified-out fiber force is as. theTwrappedhis interface mode by can failurethe beasphalt classifiedmode and [37 pulled]. as Figure the out interface 18 alongb shows with failure that some mode the of fiber the [37 asphalt]. is Figure wrapped, and 18b bythis shows the can asphalt be that viewed the and fiber pulledas the is outwrappedmatrix along failure withby the mode some asphalt of[37 theand]. In asphalt, pulledthis situation, out and along this the canwith bonding be some viewed ofbetween the as asphalt the the matrix fiber, and and failurethis thecanmode asphaltbe viewed [37 is]. strong as In the this situation,matrixenough failure to the hold bonding mode the pull [3 between7-]out. In forcethis the situation,. The fiber failure and the thesituation bonding asphalt shown between is strong in Figurethe enough fiber 18c and to presents hold the theasphalt that pull-out the is fiberstrong force. is Theenoughfractured failure to when situationhold theit is shownpullbeing-out pulled in force Figure out. The 18from cfailure presents the asphalt,situation that theand shown fiber this is in mainly fracturedFigure because18c when presents the it is fiber beingthat could the pulled fiber bond outis fromfracturedwell thewith asphalt, when asphalt, it and is but being this the pulled is pull mainly-out out becauseforcefrom isthe bigger the asphalt, fiber than couldand the this bondfracture is mainly well strength with because asphalt, of thethe fiberfiber but could theitself. pull-out bond This forcewellphenomenon iswith bigger asphalt, thancan bebut the ascribed the fracture pull to-out strength the force weak ofis strength thebigger fiber than of itself. the the f Thisiberfracture or phenomenon the strength strong ofadhesion can the be fiber ascribed between itself. toThis the the weakphenomenonfiber strengthand the ofcanasphalt the be fiber ascribed matrix. or the toG strongen theerally, weak adhesion it strengthcould between be of concluded the thefiber fiber orthat andthe the strong the failure asphalt adhesion model matrix. betweenof the Generally, fiber the- reinforced asphalt will be affected by the bonding situations between the fiber and the asphalt and itfiber could and be the concluded asphalt matrix. that the G failureenerally, model it could of the be fiber-reinforcedconcluded that the asphalt failure will model be aff ofected the byfiber the- the fracture strength of the fiber itself. bondingreinforced situations asphalt betweenwill be affected the fiber by and thethe bonding asphalt situations and the fracturebetween strength the fiber of and the the fiber asphalt itself. and the fracture strength of the fiber itself.

(a) (b) (c)

FigureFigure 18. 18.Failure Failure(a) situations:situations: (a) fiberfiber slipsslips out from(b) asphalt;asphalt; ( (b)) fiber fiber wrapped wrapped with with(c) asphalt asphalt in in the the pull-outFigurepull-out 18. process; process Failure; ( c( )csituations) fiber fiber breaks breaks: (a) in infiber the the asphalt. slipsasphalt out. from asphalt; (b) fiber wrapped with asphalt in the pull-out process; (c) fiber breaks in the asphalt. 4.4. Conclusions Conclusions 4. ConclusionsAfterAfter conducting conducting a a seriesseries ofof studiesstudies onon thethe performance and strengthening mechanism mechanism of of mixed mixed fiber-reinforced asphalt with different fiber mix ratios, the following conclusions can be drawn: fiber-Afterreinforced conducting asphalt a serieswith different of studies fiber on themix performance ratios, the following and strengthening conclusions mechanism can be drawn: of mixed 1.fiber1. Basalt-Basaltreinforced fiberfiber improvesasphaltimprove withs thethe different softeningsoftening fiber point mix and ratios, ductility the following of MFRA conclusionsbetter better than than lignin lignincan be fiber fiber. drawn:. A A lower lower 1. FMRBasaltFMR valueva fiberlue willimprove will cause causes thea worse a softening worse enhancement enhancement point and effect ductility eff onect the onof softening MFRA the softening better point than and point lignin the and ductility. fiber the. A ductility. Lignin lower FMR value will cause a worse enhancement effect on the softening point and the ductility. Lignin

Materials 2020, 13, 2520 15 of 17

Lignin fiber increases the complex modulus (G*) and the rutting factor (G*/sin δ) of MFRA more significantly than basalt fiber. FMR has more impact on the G* and G*/sin δ in higher frequency. The phase angle fluctuates with the increase of frequency, and the fluctuation range is limited. 2. Lignin fiber absorbs more asphalt than basalt fiber. The amount of asphalt that lignin fiber could absorb is 8.4 times its own weight. With the increase of basalt fiber, the oil absorption rate of the fiber mix gradually gets smaller and it is just half of that of LF when the fibers are all BF (FMR is 3:0). 3. The FMR is proposed as 1:2 according to the analysis by the efficacy coefficient method. The satisfactory comprehensive performance of MFRA is available when the FMR is 1:2. 4. It was observed from the SEM images that basalt fiber and lignin fiber interweaved with each other and lignin fiber wound around the basalt fibers, which formed a more stable three-dimensional network structure to make the mixed fibers hold more asphalt and eventually improve the performance of MFRA.

Author Contributions: Conceptualization, X.W. and C.K.; methodology, C.K.; software, X.W.; validation, P.X., Y.L. and Z.W.; formal analysis, C.K.; investigation, X.W.; data curation, C.K. and X.W.; writing—original draft preparation, X.W. and C.K.; writing—review and editing, C.K., P.X. and Z.W.; visualization, X.W.; supervision, C.K.; project administration, P.X.; funding acquisition, C.K. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (Grant Number 51908487), Natural Science Foundation for Youths of Jiangsu Province (Grant Number BK20190913), Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (Grant Number 19KJB580005) and Yangzhou University International Academic Exchange Fund (Grant Number 20190212). Acknowledgments: The authors want to thank the assistance of the Yangzhou University Test Center for providing some of the test instruments and materials. The authors also want to thank Material Editorial Office for their encouragement, support during the manuscript preparation. Conflicts of Interest: The authors declare no conflict of interest.

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